1. Field of the Invention
The present invention relates to a zoom lens. More specifically, the present invention relates to a zoom lens useful as a photographic optical system for an image pickup apparatus, such as a digital camera, a video camera, a television (TV) camera, or a silver-halide film camera.
2. Description of the Related Art
Continuous improvement on CCD (charge coupled device) or CMOS (complementary metal oxide semiconductor) image sensors used in image pickup apparatuses, such as a digital camera or a video camera, has yielded image sensors with an ever larger number of pixels. Accordingly, market forces dictate that an image pickup apparatus that includes such an improved image sensor should use an equally improved photographic lens. Specifically, it is desired a high-resolution zoom lens whose chromatic aberration has been sufficiently corrected to prevent color bleed on an image when a white light source is used as well as monochromatic aberrations, such as spherical aberration and coma, have been sufficiently corrected.
In addition, in order to enlarge the shooting area, it is also desired that a photographic lens used in the above-described image pickup apparatus should have a high zoom ratio. In particular, for a telephotographic type zoom lens, which has a high zoom ratio and a long back focus at the telephoto end, both primary achromatization and the correction of secondary spectrum should be appropriately executed for correcting chromatic aberration in order to increase the resolution.
As a zoom lens having a high zoom ratio, a positive-lead type zoom lens has been widely used. The positive-lead type zoom lens includes, in order from the object side to the image side, a first lens unit having a positive refractive power, a second lens unit having a negative refractive power, and a subsequent lens unit including one or more lens units and having an overall positive refractive power.
A zoom lens of the positive-lead type is known and has been conventionally used. One type of the positive-lead type zoom lens uses an anomalous partial dispersion material having a high dispersion as a material of a lens included in the first lens unit in order to appropriately correct chromatic aberration. More specifically, as the positive-lead type zoom lens described above, a four-unit zoom lens has been conventionally used, which includes, in order from the object side to the image side, positive, negative, positive, and positive lens units, in which the first lens unit is made of a low-dispersion material having an anomalous partial dispersion.
U.S. Pat. No. 7,505,214 and U.S. Pat. No. 7,643,228 each discuss a zoom lens whose first lens unit is made of a high-dispersion optical material having an anomalous partial dispersion to appropriately correct chromatic aberration at the telephoto end. The zoom lens discussed in each of U.S. Pat. No. 7,505,214 and U.S. Pat. No. 7,643,228 corrects chromatic aberration of the entire zoom lens system over the entire zoom area by suppressing the secondary spectrum that may occur in the first lens unit. In addition, U.S. Pat. No. 7,304,805 discusses a five-unit zoom lens including, in order from the object side to the image side, positive, negative, positive, negative, and positive lens units, which is capable of correcting chromatic aberration at the telephoto end with the first lens unit made of a high-dispersion material having an anomalous partial dispersion.
If the positive-lead type zoom lens described above is used, it becomes relatively easy to achieve a high zoom ratio on a zoom lens system whose total size is small. However, if the zoom ratio is simply increased in the positive-lead type zoom lens, chromatic aberration may greatly vary. In addition, in this case, a large amount of secondary spectrum of axial chromatic aberration may occur in the zoom area at the telephoto end.
In the four-unit or five-unit zoom lens described above, the height of incident light may become high in the first lens unit in the telephotographic area. In other words, axial chromatic aberration may arise primarily in the first lens unit. In order to suppress chromatic aberration and the secondary spectrum thereof occurring in this case, it is necessary to correct chromatic aberration and the secondary spectrum with the first lens unit, in which the height of a paraxial ray becomes high at the telephoto end.
In the zoom lens discussed in each of U.S. Pat. No. 7,505,214 and U.S. Pat. No. 7,643,228, a lens made of a high dispersion material is used in the first lens unit having a positive refractive power. Accordingly, it is necessary that the material of the negative lens included in the first lens unit thereof has a dispersion higher than the dispersion of the lens used in the first lens unit having a positive refractive power in order to achieve primary achromatism more intensely by the negative lens than by the positive lens.
In addition, in the above-described conventional zoom lens, it is necessary to assign a high negative refractive power to the negative lens to execute effective primary achromatism. Therefore, it is necessary for the above-described conventional zoom lens to use a material having a high refractive index as a material of the negative lens to correct spherical aberration and coma that may occur in the first lens unit at the telephoto end.
Thus, in the positive-lead type zoom lens, in order to suppress chromatic aberration that may occur when the zoom ratio is increased and to achieve a high optical performance for the entire zoom area, it is significant to appropriately set the lens configuration of the first lens unit having a positive refractive power. In addition, in order to suppress variation of various aberrations, such as chromatic aberration that may occur during zooming, it is also significant to appropriately set the lens configuration of the second lens unit.